To improve thermal efficiency of gasoline internal combustion engines, dilute combustion, using either air or re-circulated exhaust gas, is known to give enhanced thermal efficiency and low NOx emissions. There is, however, a limit at which an engine can be operated with a diluted mixture because of misfire and combustion instability as a result of a slow burn. Known methods to extend the dilution limit include: 1) improving ignitability of the mixture by enhancing ignition and fuel preparation; 2) increasing the flame speed by introducing charge motion and turbulence; and 3) operating the engine under controlled auto-ignition combustion.
The controlled auto-ignition process is sometimes called a Homogeneous Charge Compression Ignition (HCCI) process. In this process, a mixture of combusted gases, air, and fuel is created and auto-ignition is initiated simultaneously from many ignition sites within the mixture during compression, resulting in very stable power output and high thermal efficiency. The combustion is highly diluted and uniformly distributed throughout the charge. Therefore, the burned gas temperature and hence NOx emissions are substantially lower than that of traditional spark ignition engines based on propagating flame front and diesel engines based on an attached diffusion flame. In both spark ignition and diesel engines, the burned gas temperature is highly heterogeneous within the mixture with very high local temperatures creating high NOx emissions.
Engines operating under controlled auto-ignition combustion have been successfully demonstrated in two-stroke gasoline engines using a conventional compression ratio. It is believed that the high proportion of burned gases remaining from the previous cycle, i.e., the residual content, within the two-stroke engine combustion chamber is responsible for providing the high mixture temperature necessary to promote auto-ignition in a highly diluted mixture.
In four-stroke engines with traditional valve means, the residual content is low, and controlled auto-ignition at part load is difficult to achieve. Known methods to induce controlled auto-ignition at low and part loads include: 1) intake air heating; 2) variable compression ratio; and 3) blending gasoline with fuel that has wider auto-ignition ranges than gasoline. In all the above methods, the range of engine speeds and loads in which controlled auto-ignition combustion can be achieved is relatively narrow.
Engines operating under controlled auto-ignition combustion have been demonstrated in four-stroke gasoline engines using variable valve actuation with unconventional valve means. The following are descriptions of two such valve strategies, specifically an exhaust re-compression valve strategy and an exhaust re-breathing valve strategy. With either valve strategy, a high proportion of residual combustion products from previous combustion cycles is retained to provide the necessary conditions for auto-ignition in a highly diluted mixture. The range of engine speeds and loads in which controlled auto-ignition combustion can be achieved is greatly expanded using a conventional compression ratio.
One such valve strategy is an exhaust re-compression valve strategy. A four-stroke internal combustion engine has been disclosed that provides for auto-ignition by controlling the motion of the intake and exhaust valves of a combustion chamber to ensure that a fuel/air charge is mixed with combusted gases to generate conditions suitable for auto-ignition. In particular, this engine operates with a mechanically cam-actuated exhaust valve that is closed earlier in the exhaust stroke than normal four-stroke engines to trap combusted gases for subsequent mixing with an intake of a fuel and air mixture.
Further, a similar method of operating a four-stroke internal combustion engine has been disclosed in which combustion is achieved at least partially by an auto-ignition process. Flows of fuel/air charge and combusted gases are regulated by hydraulically controlled valve means in order to generate conditions in the combustion chamber suitable for auto-ignition operation.
The valve means used includes an intake valve controlling flow of the fuel/air mixture into the combustion chamber from an inlet passage and an exhaust valve controlling flow of exhaust combusted gases from the combustion chamber to an exhaust passage. The exhaust valve opens (EVO) at approximately 10 to 15 degrees before bottom dead center in the expansion stroke, and closes (EVC) during the exhaust stroke in a range of 90 to 45 degrees before top dead center. The intake valve is opened (IVO) later in the four-stroke cycle than usual in a normal four-stroke engine in the range of 45 to 90 degrees after top dead center during the intake stroke.
The early exhaust valve closing and late intake valve opening provides a negative valve overlap period (EVC-IVO) in which both exhaust and intake valves are closed for trapping of combusted gas, which later mixes with the inducted fuel/air charge during the intake stroke and thereby promotes the auto-ignition process. The intake valve is then closed (IVC) roughly 30 degrees after bottom dead center in the compression stroke. This is generally referred to as an exhaust re-compression valve strategy.
A similar method of operating a direct-injection gasoline four-stroke internal combustion engine has been disclosed in which combustion is achieved at least partially by an auto-ignition process. Flows of air and combusted gases are regulated by the hydraulically controlled valve means as described above. The fuel is delivered directly into the combustion chamber by a gasoline injector. The gasoline injector is said to inject fuel during either the intake stroke or the subsequent compression stroke in a single engine cycle.
Furthermore, a system and a method for operating a four-stroke internal combustion engine has been disclosed in which part load operation is achieved by an auto-ignition process. Flows of air and combusted gases are regulated by either mechanical (phase shift of a single cam or shift between two different cams) or electromagnetic valve means similar to that described above. Control of the auto-ignition process is divided into three modes depending upon the magnitude of a predetermined operating parameter. The operating parameter is indicative of either the engine load or the engine speed. The three auto-ignition combustion modes are: a gasoline reform auto-ignition combustion mode, an auto-ignition stratified charge combustion mode, and an auto-ignition homogeneous charge combustion mode.
In the gasoline reform auto-ignition combustion mode that may be selected during operation with low part load, a first fuel injection during the negative valve overlap period produces a sufficient amount of chemical reaction for promotion of auto-ignition of the fuel/air mixture produced by a second fuel injection during the subsequent compression stroke. The fuel quantity for the first injection is disclosed to be either constant or inversely proportional to the engine load. The corresponding injection timing, however, is disclosed to be either retarded in a continuous manner or held constant as the engine load increases. In the auto-ignition stratified charge combustion mode that may be selected during operation with intermediate part load, a fuel injection during the compression stroke supports auto-ignition. The injection timing advances as the engine load increases. In the auto-ignition homogeneous charge combustion mode that may be selected during operation with high part load, a fuel injection during the intake stroke supports auto-ignition. The injection timing is disclosed to be invariant against variation of engine load.
We have demonstrated a strategy for operating a direct-injection gasoline four-stroke internal combustion engine with enhanced controlled auto-ignition combustion from low to medium load. Flows of air and combusted gases are regulated by either electro-hydraulically controlled valve means (fully-flexible valve actuation) similar to that described above or a mechanically controlled valve means (phase shift of a single cam or shift between two different cams) similar to that described above. The valve means is used in conjunction with a gasoline direct injector having multiple injection capability during a single engine cycle. The injection timing of fuel injection and the proportion of fuel split, if desired, are electronically controlled. Different negative valve overlap periods and different injection strategies are required at different engine loads for optimal control of combustion phasing and hence engine performance.
Control of the auto-ignition process is divided into three stages from low to medium load. It is experimentally confirmed that for maintaining optimal auto-ignition combustion phasing throughout the stated load range, the negative valve overlap period increases with decreasing engine load. Further, during operation with low part load (Stage 1), a first injection of a fixed amount of fuel during the negative valve overlap period produces a sufficient amount of heat and chemical species that are more reactive than the fuel for promotion of auto-ignition of the fuel/air mixture produced by a second fuel injection during the subsequent compression stroke. The injection timing for the first injection retards and the injection timing for the second injection advances in a continuous manner as the engine load increases. During operation with intermediate part load (Stage 2), a first injection of fuel during the negative valve overlap period followed immediately by a second injection of fuel during the subsequent intake stroke supports auto-ignition. Optimal separation of the two injections is around 30 to 60 degrees crank angle. The injection timings of both injections retard in a continuous manner as the engine load increases. During operation with high part load (Stage 3), a single fuel injection during the intake stroke supports auto-ignition. The injection timing retards as the engine load increases. This method has been shown to be effective in extending the load range of a direct-injection gasoline four-stroke auto-ignition engine using a conventional compression ratio.
A second valve strategy is an exhaust re-breathing valve strategy. A method of operating a four-stroke internal combustion engine has been disclosed in which combustion is achieved at least partially by an auto-ignition process. Flow of fuel/air charge and combusted gases are regulated by hydraulically controlled valve means in order to generate conditions in the combustion chamber suitable for auto-ignition operation. The valve means used include an intake valve controlling flow of fuel/air mixture into the combustion chamber from an inlet passage and an exhaust valve controlling flow of exhaust combusted gases from the combustion chamber to an exhaust passage. The exhaust valve is opened for two separate periods during the same four-stroke cycle. The exhaust valve is opened for a first period to allow combusted gases to be expelled from the combustion chamber. The exhaust valve is opened for a second period to allow combusted gases previously exhausted from the combustion chamber to be drawn back into the combustion chamber. The double opening of the exhaust valve during each four-stroke cycle creates the necessary condition for auto-ignition in the combustion chamber. This is generally referred to as an exhaust re-breathing valve strategy.
We have demonstrated a method of operating a direct-injection gasoline four-stroke internal combustion engine with extended capability for controlling the auto-ignition process at low engine load. In this method, a valve strategy that employs the double opening of the exhaust valve and a single opening of the intake valve is used in conjunction with a gasoline direct injector having multiple injection capability during a single engine cycle. Both intake and exhaust valve means are hydraulically controlled. By choosing appropriately the closing timing of the exhaust valve for the first opening event and the opening timings of both the intake valve and the exhaust valve for the second opening event, different levels of in-cylinder vacuum can be generated. Higher in-cylinder vacuum leads to increased charge temperature at intake valve closing and results in improved combustion stability for a controlled auto-ignition engine.
The combustion stability of the engine is further improved with an intelligent split-injection strategy that features two injections per engine cycle. The first injection event delivers 10–30% of the total injected fuel per cycle into the combustion chamber during the early part of the intake stroke while the second injection event delivers the remaining fuel during the later part of the compression stroke. The injection timing of each injection event and the proportion of fuel split are electronically controlled. Different levels of in-cylinder vacuum and split-injection strategies are required at different engine loads for optimal control of combustion phasing and engine performance. Both demonstrations have been shown to effectively extend the low load limit of direct-injection gasoline four-stroke auto-ignition engines.
We have also demonstrated a strategy for operating a direct-injection gasoline four-stroke internal combustion engine with enhanced controlled auto-ignition combustion from low to medium load. Flows of air and combusted gases are regulated by either electro-hydraulically controlled valve means (fully-flexible valve actuation) similar to that described above or mechanically controlled valve means (phase shift of a single cam or shift between two different cams). The valve means is used in conjunction with a gasoline direct injector having multiple injection capability during a single engine cycle similar to that described above. The injection timing of fuel injection and the proportion of fuel split, if desired, are electronically controlled. Different levels of in-cylinder vacuum and injection strategies are required at different engine loads for optimal control of combustion phasing and hence engine performance. Control of the auto-ignition process is divided into two stages from low to high part load.
It is experimentally confirmed that for maintaining optimal auto-ignition combustion phasing throughout the load range, the required in-cylinder vacuum decreases with increasing engine load. Further, during operation with low part load, a first injection of 10–30% of total injected fuel during the early part of the intake stroke promotes auto-ignition of the fuel/air mixture produced by a second fuel injection during the subsequent compression stroke. The injection timing for the first injection retards and the injection timing for the second injection advances in a continuous manner as the engine load increases to avoid excessive smoke emission. During operation with intermediate and high part loads, a single fuel injection during the intake stroke supports auto-ignition. The injection timing retards as the engine load increases to avoid excessive smoke emission. This method has been shown to be effective in extending the load range of a direct-injection gasoline four-stroke auto-ignition engine using a conventional compression ratio.
The above descriptions depict our methodologies in operating direct-injection controlled auto-ignition gasoline engines over practical speed and load ranges under steady state operations. For successful HCCI engine operation with transient, control parameters other than spark, injection, and valve timings are important. For example, under a given injection and valve strategy, controlled auto-ignition combustion is greatly affected by the delivered air-fuel ratio. Hence, control of the air-fuel ratio in a gasoline direct-injection controlled auto-ignition engine is desirable.